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Introduction: To date, more than a dozen sets of key equipment for Shell gasification – gasifiers – have been introduced in China, with a coal feeding rate of 1,000 t/d to 2,000 t/d. Five units have been put into operation; the longest operating cycle for a gasifier that has been put into use is 56 days, while the longest operating cycle for similar units abroad is one year. At present, the operation of several units is not very stable. The main reasons are: (1) the gasification furnace has a complex structure, numerous control points, and is difficult to operate ； (2) The operators are not skilled enough; they need to spend more time learning and mastering the introduced technology ； (3) The high failure rate of the grinding system and slag removal system leads to forced shutdown of the furnace. The Shell furnace is configured in a single series, and the long-term operation of the gasifier is key to the operation of the entire plant. Therefore, understanding the structure and characteristics of the gasification furnace is key to ensuring the long-term operation of the entire system. 1 Brief description of the Shell gasification process: The raw coal transported from the coal yard via belt conveyors is dried and ground in the coal grinding unit; the coal powder is then sent, under certain pressure, to the coal burner in the gasification furnace through a coal powder pressurization delivery unit. In a gasification furnace at approximately 4 MPa and 1500°C, coal powder burns in reaction with O2 and steam, thereby producing syngas as well as ash and fly ash. The syngas flows out from the top of the gasification section, and is quenched using the “cold” syngas from the wet scrubbing section to reduce its temperature to around 900°C. Subsequently, in the syngas transfer section, the gas return section, and the syngas cooling section, the temperature is further reduced to around 350°C, after which it flows out from the bottom of the syngas cooler. Most of the various non-combustible components are discharged from the bottom of the gasification furnace in the form of slag; they are rapidly cooled and transformed into glass-like particles with an average particle size of about 1 mm. Along with water, these particles are discharged from the bottom of the slag tank out of the gasification furnace and enter the slag treatment system in the slag treatment unit. A small amount of slag exists in the form of fly ash; after passing through the rapid cooling section, the conveying section, and the syngas cooling section, it is discharged from the gasifier along with the syngas and collected in the downstream fly ash removal system. The internal components of the gasifier itself consist of a membrane-type water wall and a tube-type cooler, which are installed within the entire gasifier housing. In such an internal component, a forced cooling water circulation is maintained to absorb heat and generate medium-pressure steam. 2 Structure of the gasification furnace: The shell gasification furnace consists of a pressure-bearing shell, internal components, and auxiliary equipment (see Figure 1). It is a composite device that integrates moving and stationary components, as well as processes such as combustion, reaction, heat exchange, and quenching. It mainly includes 81 sets of equipment (see Table 1). The materials used for the main components of the gasifier are listed in Table 2. Based on their functional roles in the process, gasification furnaces can be divided into 6 sections: the gasification reaction section, the quenching section, the gas transfer section, the gas return section, the cooling section, and the auxiliary equipment. The gasifier can be divided into 3 parts based on its mechanical structure: the shell, the internal components, and the auxiliary equipment. 2.1 Vaporization reaction section The vaporization reaction section is mainly composed of a pressure-bearing shell, an internal slag pool, a thermal skirt, a slag baffle, and the membrane wall of the reaction section. The pressure vessel is made of Cr-Mo heat-resistant steel, with a 40-mm-thick layer of refractory material 130RGM sprayed on its inner wall. The refractory material is supported and fixed by a \"turtle shell mesh\" welded to the inner wall, thereby preventing high temperatures in emergency situations and protecting the metal of the vessel from thermal damage. The internal slag pool is made of Incoloy alloy, while the thermal skirt consists of a cylindrical structure formed by welding INCOLOY alloy Ω tubes, in order to protect against corrosion caused by high temperatures, slag water, and condensate. The slag baffle and the membrane wall of the reaction section, made of E1320 material, are constructed by welding Cr-Mo heat-resistant steel tubes together with fins; insulation pins are welded to the inner surface of these membrane walls to hold the refractory material SiC75P in place, with an average thickness of 14 mm for this refractory material. 2.2 Quenching Section The quenching section is mainly composed of the quenching section housing, the quenching zone, and the quenching tubes. The outer shell of the rapid cooling section is made of Cr-Mo heat-resistant steel and lined with refractory material, serving the same purpose as the shell of the gasification section. The quench zone consists of two functional areas: one in which the syngas, which has been cooled and filtered by the wet scrubbing unit (at around 200°C), is mixed with the high-temperature syngas flowing out from the top of the reaction section (at around 1500°C), in a ratio of approximately 1:1; as a result, the temperature of the mixed syngas drops sharply to around 900°C ； The second is the \"quench bottom cleaning zone\", where high-pressure nitrogen is fed into this area and sprayed through 192 nozzles in order to reduce or remove the ash and slag accumulated at the outlet of the gasification section. All components in the quench zone are made of INCOLOY alloy to withstand high temperatures and corrosion. The quench tube E1301 is made of Cr-Mo heat-resistant steel and has a tube-fin-tube (membrane wall) structure; the syngas is further cooled through this quench tube. 2.3 The gas transfer pipe section is approximately 1 mm in diameter; it together with the slag water is discharged from the bottom of the slag tank into the gasifier, and then into the slag treatment system of the slag treatment unit. A small amount of slag exists in the form of fly ash; after passing through the rapid cooling section, the conveying section, and the syngas cooling section, it is discharged from the gasifier along with the syngas and collected in the downstream fly ash removal system. The internal components of the gasifier itself consist of a membrane-type water wall and a tube-type cooler, which are installed within the entire gasifier housing. In such an internal component, a forced cooling water circulation is maintained to absorb heat and generate medium-pressure steam. 2 Structure of the gasification furnace: The shell gasification furnace consists of a pressure-bearing shell, internal components, and auxiliary equipment (see Figure 1). It is a composite device that integrates moving and stationary components, as well as processes such as combustion, reaction, heat exchange, and quenching. It mainly includes 81 sets of equipment (see Table 1). The materials used for the main components of the gasifier are listed in Table 2. 2.4 Gas return section The gas return section is mainly composed of the gas return section housing and internal components. The gas return section is also made of Cr-Mo heat-resistant steel, with its inner wall coated with refractory material, serving the same purpose as the gasification section. The inner component (E1303) is a membrane wall structure formed by welding Cr–Mo heat-resistant steel tubes and fins alternately. 2.5 Gas Cooling Section The gas cooling section mainly consists of a housing, a medium-pressure steam superheater (E1306), a second-stage evaporator (E1303B), and a first-stage evaporator. One of these evaporator sections is further divided into 2 tube banks (E1303C/D). The gas cooler shell is made of Cr-Mo heat-resistant steel, with a refractory material sprayed on its inner wall; it serves the same purpose as the gasification section. The medium-pressure steam superheater consists of a coiled tubular structure formed by welding Incoloy alloy steel tubes and fins alternately; it features 6 tubes of different diameters nested together, allowing these tubes to expand freely downward. The first and second stage evaporators are constructed by welding Cr-Mo heat-resistant steel tubes and fins alternately, with a structure similar to that of the medium-pressure superheater. The two-stage evaporator consists of 6 cylinders of different diameters nested within one another, while the single-stage evaporator has 5 cylinders of different diameters nested within one another. Surrounding the medium-pressure steam superheater and the first and second stage evaporators is an outer cylinder, which also serves as the wall of the medium-pressure evaporator. The wall is a membrane wall structure formed by alternating welding of Cr-Mo heat-resistant steel tubes, fins, and tubes. 2.6 Auxiliary Equipment 2.6.1 Hammer The hammer is a set of equipment manufactured by specialized manufacturers; it mainly consists of a cylinder and a vibrator, which are connected together through the flanges on the gasifier housing. The vibration guide rod is closely connected to the membrane wall, as well as the knocking points of the evaporator and superheater. Its main function is to prevent ash accumulation inside the components. A total of 58 knocking devices are installed in the gasification furnace; since the inner walls of the reactor and the gas transfer pipes are lined with refractory materials, knocking devices are not installed at these two locations to prevent the refractory materials from falling off. 2.6.2 Coal Burner The coal burner is manufactured by specialized producers, and its main function is to feed a mixture of coal powder, steam, and oxygen into the gasification furnace. 2.6.3 Startup, Ignition Nozzles and Their Insertion Devices The startup, ignition nozzles and their insertion devices are complete sets of equipment manufactured by specialized manufacturers; their function is to raise the temperature and pressure before coal powder is fed into the gasification furnace. 2.6.4 Flame Monitor The flame monitor is manufactured by specialized producers, and its main function is to allow observation of the ignition and combustion conditions from outside the gasification furnace. 2.6.5 Constant-force hanger: A constant-force hanger is a complete set of equipment manufactured by specialized manufacturers; its function is to support the weight of the gas cooler in the gasification furnace, allowing it to expand freely as the furnace expands when it is in operation. 2.7 Other components: An “annular space” is formed between the membrane wall of the internal components of the vaporization furnace and its outer shell. The membrane wall is divided into 4 sections, which are connected together by 3 expansion joints to allow free thermal expansion of the internal components. Two sealing partitions are installed above the thermal skirt and above the medium-pressure steam superheater, to prevent hot syngas from entering the “annular space” and causing the shell to overheat. To ensure pressure balance between the \"annular space\" and the syngas space, 120 circular holes with a diameter of φ53mm are provided on the bottom plate of the quench section. All distribution pipelines such as circulating water pipelines, nitrogen pipelines, and steam pipelines are arranged within the \"ring space\". Multiple guide points are welded to the outer shell to ensure that the entire membrane wall can expand freely. 3 Characteristics of the gasifier 3.1 Due to the use of a fluidized bed gasification process, it is possible to achieve \"slagging\" gasification at high temperatures (around 1500°C), resulting in a high carbon conversion rate. 3.2 Due to the high temperature, a high content of carbon monoxide in the crude syngas can be ensured. 3.3 During the operation of the gasification furnace, the layer of ash that solidifies on the membrane wall in the gasification reaction section can protect the inner wall of the furnace, preventing it from being eroded by molten slag, thereby achieving an effect of using slag to counteract slag. 3.4 Due to the radial installation of the coal burners at a small angle (about 4.5°), an eddy current motion is generated in the reactor as a result of the airflow distribution. This motion improves the separation of slag, ash from syngas, and prevents a large amount of fly ash from being carried along. 3.5 During the operation of the gasification furnace, the ash layer on the furnace wall also helps to prevent fluctuations in the heat load that occur as a result of operational variations in the furnace. Because once there is a variation in operations, the first reaction of the solidified ash layer is to melt or thicken. 3.6 Due to the use of the coal powder gasification process, gasification is thorough, which means that the requirements for the type of coal are not very high. 3.7 Since the gasifier connects the gasification section and the gas cooler together through gas pipelines, the equipment structure becomes complex and its weight increases, which results in longer manufacturing and installation times as well as increased difficulties in production. 3.8 The increase in gasification temperature requires higher-grade materials for equipment manufacturing; Incoloy and Inconel alloys are used more frequently, which increases the complexity of production and raises costs. 3.9 Due to the use of Ω tubes in the design of the gasification furnace, it is difficult to find corresponding suppliers in China, which increases the challenges associated with local production of equipment materials and spare parts in the future. 3.10 Due to the overly complex structure of the gasification furnace and the large number of control points, it requires a high level of technical skill from operators and maintenance personnel.